Make tabs sortable

[rynobax]

I keep trying to do this and it doesn't work, so I thought I would implement it 🙂

Two questions about the implementation details:

• In chrome the new tab button disappears when you are dragging tabs, so that's how I implemented it here. However the playground tab is a little more visible, so I was thinking about maybe leaving it visible and making it the right border of the draggable area. Anyone have thoughts on what looks best?
• In chrome dragging a tab also makes it the active tab. I was having trouble getting the dragged tab to look "active". I think I know how I could do it, but I'm not sure how much people expect this behavior, so I thought I would ask first if it should be implemented.

Also, the snapshot tests are failing. Should I go ahead and update them in the PR?

[CLAassistant]

All committers have signed the CLA.

[huv1k]

Wow! Nice work, I really like it. I tested and its working nicely. The whole topbar needs little bit change, because currently there is a problem with tabs when they overflow on the right side. It would require to rethink it more. In my opinion, it should change to active tab because you want to "do" something with that tab.

[rynobax]

Added two fixes:

• Dragging a tab will cause it to be active
• If you have enough tabs to make the tab bar scrollable, it will scroll if you drag a tab towards the edge

I think this is good to go, but let me know if you think of anything else that should be changed.

[huv1k]

Thanks for your first contribution! It will be in next release 🚀